The drop in the efficiency of radiation-emitting semiconductor chips as current density increases has been a long-term problem, which, inter alia, limits brightness per given area and thus radiance. Semiconductor chips connected in series and/or stacked on one another may solve this problem in part. With semiconductor chips grown monolithically over one another and which emit radiation in the visible wavelength range, the large band gap results in extremely high dopant concentrations, wherein at the same time the dopants have high activation energy. This leads to a drastic reduction in effective charge carrier density and to ohmic losses, for example, at tunnel junctions, which can be integrated into the semiconductor chip.
It is conventionally known to achieve high radiances with semiconductor chips stacked vertically one above the other. In this case, the individual semiconductor chips are connected together by a crystalline tunnel junction or by bonding layers. Such stacked semiconductor chips may be operated at high efficiency, wherein the same quantity of light can be generated compared to single semiconductor chips but with such stacked semiconductor chips being operable at lower current densities. However, these vertically stacked semiconductor chips display an elevated operating voltage, since ohmic losses at the tunnel junction cannot be avoided. In addition, radiation efficiency is impaired by internal absorption in the semiconductor chip, even if the tunnel junction is operated in a loss-free manner.
European patent publication EP 1601026A1, which has counterpart U.S. Pat. No. 7,459,727, describes, for example, an optoelectronic component which comprises a layer stack comprising two semiconductor layer sequences of opposing orientation which are monolithically integrated. The semiconductor layer sequences each have a first semiconductor region and a second semiconductor region, between which an active zone is in each case arranged. A contact layer is arranged between the individual semiconductor layer sequences.
German patent publication DE 102008053731 describes an optoelectronic semiconductor chip with two active zones. In this case, the semiconductor chip is formed of two separate semiconductor bodies, which are joined together mechanically by a bonding layer.